On the nature of moving magnetic feature pairs around sunspots Zhang et al., 2003, A&A, 399,755 2003 May 26 Zasshikai short-talk Junko Kiyohara

1. Introduction Moving magnetic features (MMFs) are small magnetic features that move away from a sunspot to the periphery of the surrounding moat.

・ They are transported by the outward moat flow. ・ There are roughly two types of MMFs: unipolar and mixed polarity ( Harvey and Harvey 1973) ・ Several models are proposed. Harvey and Harvey (1973) : Magnetic flux is removed from the sunspot at the photospheric level. Wilson (1973) : Thin flux tube is detached from the sunspot well below the surface. Wilson (1986) : Two MMFs form a pair connected by an O-loop. Ryutova et al. (1998) : Ω loops emerging from below etc. Harvey and Harvey (1973) Wilson (1973) Figure 1 in Yurchyshyn et al.(2001)

28 MMF pairs were studied using BBSO magnetograms. Yurchyshyn et al. (2001) 28 MMF pairs were studied using BBSO magnetograms. MMFs are not randomly oriented. The magnetic elements having the same polarity as the sunspot are located further from the sunspot There is a correlation between the orientation of the MMF bipoles and the twist of the sunspot superpenumbra, deduced from Halpha images. ⇒　MMFs are still attached to the sunspot’s superpenumbral canopy magnetic field. In this paper, a larger sample of MMFs than Yurchyshyn are studies to test whether their results are statistically significant and to further constrain properties of MMFs.

Observations and analysis Data : MDI high-resolution magnetograms (spatial resolution 0”.625 temporal resolution 1 min) Well-isolated MMF pairs which are reliably identified on tens of successive magnetograms are selected. NOAA8375 : 93 pairs ( 40 hours ) location : N18W06 NOAA9236 : 51 pairs ( 48 hours ) location : N23W05

Definition of the parameters : α: the angle of the axis of an MMF bipole with respect to the radial direction from the sunspot center. β: the angle between the line connecting the center-of gravity of an MMF pair with the sunspot center and the line from sunspot center to the north.

Properties of moving magnetic features Magnetic flux : mean flux per MMF pair 3.6×1018Mx Location of first appearance : 1000－5000km(2-7arcsec) from the penumbral boundary Motion : Averaged velocity over the whole lifetime is 0.45 km/s Lifetime : 4 hours Separation between the two members of an MMF pair : 1100-1700km This separation remains almost unchanged, even decreases slightly as the MMF pairs move outwards.

Orientation of MMF bipoles : The member of an MMF pair further from the sunspot has the polarity of the parent sunspot in 85% of the cases. The orientations of MMF pairs are associated with the twist of the sunspot superpenumbra deduced from Halpha images. NOAA8375 α= 38° NOAA9236 α= -25° For AR8375 most of the MMF pairs are distributed at β between 0 and 90°. 　 The line with β= 40°connects the main spot with a smaller spot. The MMFs pairs with initially 10°<β<40°, δβ>0 40°<β<60°, δβ<0

A qualitative model MMF pairs are formed when the field lines in a small part of the magnetic canopy dip down to produce a U-loop. 1. An instability caused by the presence of a shear flow at the base of the canopy ( Holzwarth & Schussler 2002 ) 2. flux pumping by granulation　( Thomas et al. 2002 ) 3. Evershed flow in sunspot canopy Evershed flow is a nearly horizontal intermittent flow of matter directed radially outward in the penumbra. Within the penumbra a packet of dense outward flowing gas is prevented from sinking by magnetic forces. At the edge of the penumbra this supporting force disappears and this gas then sinks. 　⇒　A U-loop is created. subemergence of field and a downflow near the penumbral edge: Westendorp Plaza et al. (1997) Schlichenmaier & Schmidt (2000) Mathew et al. (2002)

・ Distributions of the MMFs The U-loop model is supported by various　observations. ・ Distributions of the MMFs ・ Orientation of the axes of MMFs ・ MMFs are not visible in the choromospheric He I 10830Å(Penn & Kuhn 1995) , but are well seen in the upper photosphere ( Shine & Title 2001) 　・ The separation does not change significantly with time 　・ Pores are not surrounded by MMFs. ⇒　 A penumbra is necessary to produce MMFs.

Conclusion This work confirms the results of Yurchyshyn et al.(2001) , as well as revealing further systematics in the properties of MMFs. Observations shows that MMFs are the intersections with the solar surface of U-loops produced by localized dips of the magnetic canopy. It is unclear whether these results can be applied to MMFs around the older spots. Further properties of MMFs, for example, flows within them, their field strengths and field inclinations should be studied.